1. Technical Field
The present invention relates in general to improved methods and systems for managing datasets stored within storage subsystems in a data processing system and in particular to improved methods and systems for accessing desired datasets stored within storage subsystems in a data processing system. Still more particularly, the present invention relates to improved methods and systems for accessing desired datasets within a storage subsystem utilizing non-address attributes set forth within a data request.
2. Description of the Related Art
Data processing systems frequently include large scale storage devices, such as Direct Access Storage Devices (DASD) which are located externally to a host computer system and sometimes at significant distances from the host computer system. Communication from the host computer system to the DASD is typically accomplished over signal cables, extending between the DASD and its control unit and connecting the DASD devices to the host computer system.
Current technology provides DASD units with several separate disks, all rotating on a single spindle. These disks or platters are accessed by head disk assemblies with a transducing head providing access to one surface of each disk. There may be, for example, nine platters in a disk drive providing sixteen surfaces with one of the surfaces used for maintaining accurate tracking capability. In such units there are fifteen usable surfaces for data and when all heads are correctly positioned a cylinder of fifteen physical, recording tracks may be accessed.
DASD units frequently use a so-called "Count-Key-Data" architecture (CKD) where records written on a track within a DASD unit are provided with a count field (an identification), an optional key field and a data field. A record may occupy one or more units of real storage. A "dataset" is a logical collection of multiple records which may be stored on contiguous units of real storage or which may be dispersed. Data is then stored and/or retrieved from a DASD using write and read requests which are issued by the host system. The mechanism which enables host systems to retrieve data which has previously been stored on a disk is the "data disk address." Therefore, when issuing a write request, the host system specifies where on the DASD storage subsystem the data should be placed. Later, if the host system wishes to retrieve this data it will issue a read request utilizing the same address.
Thus, data stored on a disk within a storage subsystem is always associated with a unique data descriptor which identifies that data. In a write request the host specifies the data descriptor together with the data to be stored. In a read request the host specifies the data descriptor of the data it wishes to receive. In response to such a request the DASD subsystem will send the referenced data back to the host. For purposes of explanation herein such read requests which utilize data descriptors are referred to as "specific read requests."
Those skilled in the art will appreciate that it would be advantageous to permit a host system to retrieve data from a DASD subsystem on a basis other than the data descriptor. For example, in cases where the host system requests a large amount of data occupying many disk tracks the efficiency of transferring that data to the host might be enhanced if the order of the transfer is adjusted in order to minimize both seek time and latency time within the data storage subsystem. This is not generally possible since host systems do not know the head location within the DASD subsystem and thus are not able to issue specific read requests which would minimize the disk seek and latency time.
Additionally, many systems exist which are utilized to create so-called "backup" copies of data. In an incremental backup copy only that data which has been modified since the previous copy need be transferred to the host system. In such cases, the host does not initially know what data has been modified and thus the host can not issue specific read requests without issuing a query to the storage subsystem to determine the data descriptors for the data which has been updated since the previous copy. In view of the above, those skilled in the art will appreciate that it would be desirable to permit a host system to retrieve data from a storage subsystem by specifying certain attributes of the data rather than the actual data address.